1) Field of the Invention
This invention pertains to the field of wavelength division multiplexed optical communication systems and, more particularly, to method of performing bit error rate tests for a wavelength division multiplexed optical communication system having a plurality of optical communication channels.
2) Background of the Related Art
Optical devices are increasingly being used in communication and information systems. An optical communication system, as used herein, refers to any system which uses optical signals to convey information across an optical waveguiding medium, such as an optical fiber. Examples of such systems include, but are not limited to, telecommunications systems, cable television systems and local area networks (LANs).
In the past, optical communication systems were designed to communicate data on an optical fiber via an optical communication channel having a single wavelength. To convey information to and from multiple sources and/or destinations, time division multiplexing (TDM) was frequently employed to share the single-wavelength channel. If multiple communication channels were desired, multiple fibers could be used.
More recently, wavelength division multiplexing (WDM) has been employed in optical communication systems to increase the information capacity of existing fiber networks. A WDM optical communication system employs multiple optical communication channels in a single fiber, each channel carrying a different optical signal operating on a different optical wavelength and transmitted over a single optical waveguide, or fiber.
As the demand for communications has increased, WDM optical communication systems are being developed with more and more optical communication channels. There are now WDM systems which have 80-100 separate optical communication channels each operating on a different optical wavelengths on a single optical fiber.
One standard measure of the quality of an optical communication system is the specified bit error rate (BER) for an optical communication channel in the system.
FIG. 1 shows a prior art arrangement for testing the BER for an exemplary optical communication system comprising two optical communication network elements 105, 107. Each optical communication network element 105, 107 comprises N optical transmitters 120 and N optical receivers 130 for communicating over N optical communication channels. Each optical communication channel has a specified BER value. If the BER for an optical communication channel is greater than the specified BER value, then the optical transmitter/receiver pair 120/130 for that optical communication channel is not within specification and corrective measures are in order. A BER tester 140 is used to measure the BER for each optical communication channel to test whether or not the BER is within specification. Typically, the BER tester 140 comprises a BER test signal generator and a BER detector.
To test the BER for an optical communication channel, i, the BER test signal generator is connected to the optical transmitter 120 Txi of the first optical communication network element 105. The output of the optical transmitter Txi is connected via an optical waveguide or optical fiber 115 to the corresponding optical receiver Rxi of the second optical communication network element 107. The output signal from the optical receiver 130 Rxi of the second optical communication network element 107 is provided to the BER detector in the BER tester 140.
The BER test signal generator of the BER tester 140 supplies a BER test signal to the optical transmitter 120 Txi of the first optical communication network element 105. The BER detector receives the BER test signal from the optical receiver 130 Rxi of the second optical communication network element 107 and detects and counts any bit errors produced by passing the BER test signal through the optical communication channel i to produce a measured BER for the optical communication channel i. Finally, the measured BER for the optical communication channel i is compared against a specified BER value for the optical communication channel to determine whether or not the optical communication channel i is within specification.
To test BER for the WDM optical communication system, the optical communication channel BER test is repeated for each optical communication channel i, in the optical communication system, where iε(1, N). If the measured BER for each of the optical communication channels is less than the specified BER value, then the optical system meets its BER requirements and passes the BER test. If one or more of the optical communication channels has a measured BER which is greater than the specified BER value, then corrective measures are required, including troubleshooting and repairing the optical transmitter/receiver pair 120/130 for the optical communication channels which failed the BER test.
The above-described prior art method works well when the number of optical communication channels is small and the specified BER values are relatively large.
However, there is a problem in testing a WDM optical communication system having many optical communication channels, each operating with a low specified bit error rate. To accurately and reliably measure a bit error rate for an optical communication channel, it is generally considered necessary to communicate enough data through the optical communication channel so that, at the specified BER, an average of ten errors will be produced. As technology has improved, WDM optical communication systems with lower and lower specified BERs are being produced. WDM optical communication systems have been developed which specify an optical communication channel BER of less than 10E-15.
Accordingly, to measure the BER for an optical communication channel having a specified BER is 10E-15, it is necessary to communicate at least 10E16 bits through the channel, which will on average produce ten errors. In a WDM optical communication system specifying an optical communication channel BER of less than 10E-15 and a data rate of 2.5 Gbps, the BER measurement for a single optical communication channel takes more than 46 days. In the case of a WDM optical communication system having forty (40) optical communication channels each having a specified BER of less than 10E-15 at a specified data rate of 2.5 Gb/s, it would take over 5 years to test the BER for the entire system, if each channel is tested sequentially in time. Clearly, a test taking so long is completely unacceptable.
Alternatively, it is possible to use 40 separate BER testers to test the 40 optical communication channels in parallel at the same time. However, the BER testers are somewhat expensive, and this approach is very costly.
Accordingly, it would be advantageous to provide an improved method of testing BER for a WDM optical communication system having a plurality of optical communication channels. It would also be advantageous to provide such a method wherein when one or more of the optical communication channels has an actual BER which exceeds a specified BER value, those optical communication channels are identified so that the optical transmitter(s) and/or optical receiver(s) for those optical communication channels can be troubleshot and repaired. Other and further objects and advantages will appear hereinafter.